This invention relates to coatings which provide an article with improved hot corrosion resistance and, more particularly, to a coating for use in connection with a Fe, Co or Ni based substrate and comprised of a coating matrix interdiffused with an aluminum filler. The invention herein described was made in the course of or under a contract, or a subcontract thereunder, with the United States Department of the Air Force.
Components of certain energy conversion apparatus, such as gas turbine engines, operating in an oxidizing atmosphere in the temperature range of about 1300.degree.-1800.degree.F, have suffered degradation from environmental exposure. A principal mode of attack is hot corrosion. It can occur when ingested airborne salt, particularly in marine environs, combines with fuel sulfur. Sodium sulfate can form as a condensate on apparatus parts such as in the turbine and can aggressively attack component alloys or their coatings.
Modern turbine engines are constructed of superalloys based on the transition triad elements Fe, Co and Ni and alloyed to have some inherent resistance to corrosion. However, to extend the life of such alloys, protective coatings have been used. One such class of coatings is the aluminides. These generally are formed by a high temperature interdiffusion reaction at the interface between aluminum, applied in some form, and of the superalloy substrate. A variety of coating processes involving such reaction have been widely reported and are commercially available. Another class of reported coatings is the MCrAlY vapor coatings in which M is the base metal element. These are alloys of Fe, Co or Ni base alloyed with Cr, Al and Y and deposited by vacuum vapor condensation on a substrate surface. Such vapor coatings have been shown to have certain advantages in providing extended life to articles such as turbine parts. However, they are relatively costly to produce and require relatively expensive manufacturing equipment.
An additional requirement of such coatings is that they do not degrade the superalloy properties either through process effects or physical surface effects. In this regard, soft, ductile coatings are preferred to hard, brittle coatings because they yield more to rapid thermal cycling of the type found in a turbine. Furthermore, they are not prone to surface cracking and stress concentrations which can degrade fatigue properties.